The incidence of breast cancer on Long Island has been increasing over the past few years. Both Nassau and Suffolk Counties rank among the highest in New York State, which itself has a higher incidence than the U.S. national average.The Long Island Breast Cancer Study, a large case-control study conducted in the mid 1980's found that standard risk factors explain only a fraction of the excess incidence. Therefore, we are proposing this study to examine several environmental factors which have been hypothesized to be associated with breast cancer. Recent work with laboratory animals has suggested that electromagnetic radiation (EMF) of about 60 hz might be associated with breast cancer by inhibiting secretion of melatonin by the pineal gland. By reducing melatonin secretion, it could be increasing estrogen production, a well established risk factor for breast cancer. In addition, several recent small-scale studies found higher levels of environmentally widespread organochlorine compounds in breast adipose tissue of breast cancer cases than controls. These recent reports have created much concern among Long Island women, given the high usage of electromagnetic radiation and the previously high usage of organochlorine pesticides on Long Island. We are therefore proposing to do a case-control study comparing breast cancer cases to control women. The study will be limited to those women residing in the same house for at least fifteen years. EMF measurements will be made both inside and outside these residences, and chemical analyses will be done on serum from these women. Household dust samples will be collected and analyzed on a random sample of women. Levels of organochlorine pesticide residues will be compared between serum and dust samples. In addition, a detailed questionnaire will be administered to each participant in order to gain information regarding environmental exposures and standard breast cancer risk factors. It is hoped that this study will help to clarify the reasons for the high breast cancer incidence on Long Island.